The invention relates to a cascode current-source arrangement comprising a first and a second transistor whose collector-emitter paths are arranged in series between a first terminal and a common second terminal, and a third transistor connected as a diode, whose base is connected to the base of the second transistor and whose emitter is connected to the common second terminal.
Such cascode current-source arrangements are generally applicable in integrated circuits and are in particular suitable for use in amplifier circuits as described in U.S. patent application Ser. No. 703,146 filed simultaneously with the present application.
Such a current-source arrangement is known from FIG. 2 of U.S. Pat. No. 4,345,217. The collector current of the second transistor is defined by connecting a diode-connected transistor in parallel with the base-emitter junction of this second transistor. The collector current of the second transisttor then also flows through the collector-emitter path of the first transistor, whose base is at a reference voltage. As a result of this reference voltage the collector-emitter voltage of the second transistor is then also constant. In practice, this reference voltage is generally generated by arranging a second diode-connected transistor in series with the first diode-connected transistor, the base of said second diode-connected transistor being connected to the base of the first transistor. The collector-emitter voltage of the second transistor is then equal to one base-emitter voltage. A disadvantage of this arrangement is that, if the collector of the second transistor is used as a signal input, the lowest voltage attainable on the collector of the first transistor is equal to the sum of the base-emitter voltage, which appears across the collector-emitter path of the second transistor, and the saturation voltage of the first transistor. However, in order to obtain a maximum voltage swing the collector voltage of the second transistor must be as low as possible. This is of particular importance if the current-source arrangement is used with low supply voltages. Another disadvantage of this arrangement is that owing to the difference in the collector-emitter voltages of the diode-connected transistor and the second transistor, the current flowing through the second transistor is not exactly equal to the current through the diode.